Taunts: I would like to see an extra taunt. One taunt for each direction on the d pad would be a little thing that could make the gameplay a little more enjoyable.
Clones: Perhaps the biggest problem was Brawl was the clones. Ganondorf needs actual moves, Fox doesn't need TWO clones, etc.
Online Play: Just make it faster, that is all.
Downloadable Content: This was a rumor for Brawl, but failed to happen. It would be nice if a few stages and characters could be added. Even just some extra trophies and stickers would be nice. I'm sure whatever they make, Nintendo could charge people and then both the company and players would win.
Classics: The game demos were a nice little add-on to the series, but there is great room for improvement for these. They could make more on my first idea, so listen up Reggie: Put demos on the game, but make them able to purchase right there and to play on the game. The other suggestion I have is make the demos longer. I'm not saying 2 hours, but come on, 30 seconds for Ice Climber? That's barely enough to press start. How about 2 or 3 minutes for games like Super Mario Bros. and Ice Climber, 10 minutes for games like Super Metroid, and 15 minutes for games like Legend of Zelda Ocarina of Time.
Game Modes: The addition of a game mode that told a story was a great step up from the "adventure mode" in melee. That said, I think we need some enhancements. First of all, how about enemies that actually come from real games instead of those stupid looking things that were called "enemies"? Maybe making more of the unlockable characters harder to get.
Intro: The intro was horrible and a disgrace to the Smash Bros. franchise, mainly because it used footage of the Subspace Emissary. This killed 2 birds with 1 stone to me. First, it spoiled the surprise of unlocking both Ness (my personal favorite) and Marth, along with many secrets of the Subspace Emissary. Second, it wasn't inviting like the first two games in the series were. The intro should show the character's in their element. Otherwise, who (other than extreme hardcore fans) would ever know what game Lucas and the Ice Climbers are from?